1. Field Of The Invention
This invention relates generally to process and apparatus for dewatering aqueous clay suspensions by filtration thereof.
2. Description Of The Related Art
In the treatment of clay materials for commercial use, it is conventional practice to subject the raw mined clay to a plurality of processing steps, such as fractionation, bleaching, and removal of magnetic and paramagnetic impurities, etc., where the clay being processed is in the form of an aqueous clay suspension or slurry. The aqueous suspension form of the clay material is advantageous from the standpoint of facilitating its contact with treatment chemicals as well as transport and storage of the clay material. Regardless of the specific sequence and character of the processing steps, however, it generally is desirable to increase the solids levels of the aqueous clay suspensions above the concentrations utilized in the aforementioned processing steps for transport, storage and end use.
Accordingly, it is conventional practice to dewater the aqueous clay suspension following the aforementioned processing steps.
For dewatering of aqueous clay suspensions, rotary vacuum filters have come into widespread usage. Such filters enable the aqueous clay suspensions to be dewatered to solids levels on the order of about 56-63 percent solids depending on the conditions employed during the dewatering step. Nonetheless, there has been a continuous and ongoing effort to improve the dewatering capability of rotary vacuum filters. Much of the effort to improve dewatering has been focused on applications where the clay suspension is spray dried downstream of the filter, for example, since a 1-2% increase in the weight percent of solids can significantly lower the cost of energy required to operate the spray drying process.
A variety of methods and means have been attempted. For example, Komline-Sanderson brochure KSB-5L0-7902 (copyright 1977) by the Komline-Sanderson Company, describes optional accessories and modifications for the Komline-Sanderson Flexibelt Filter as including a compression and wash assembly. For example, such assembly consists of a support frame, one floating roll, two compression rolls, a series of belt rolls, a belt guide mechanism, two cake wash drip pipes, an excess wash trough and a compression and wash belt. The purpose of this assembly, as stated in the brochure, is to allow more uniform washing of the filter cake after it is formed, to reduce cake cracking and to reduce the moisture content of the cake by compression.
Others have attemped to increase the solids of the clay by increasing the vacuum inside the filter drum. However, this increases the passage of solids through the filter cloth or medium and additionally tends to decrease filtration efficiency by blinding the filter.